1. Field of the Invention
The present invention relates to a shaped article of a crosslinked elastomer. More particularly, the present invention is concerned with a novel shaped article comprising an elastomer which is crosslinked with at least one crosslinking agent at a crosslink density continuously decreasing from each surface toward the interior of the shaped article, so that the shaped article is not only characterized with excellent surface non-stickiness and low friction properties, but is also capable of maintaining these properties for a long period of time.
2. Discussion of Related Art
A rubber elastomer such as a natural rubber elastomer or a synthetic rubber elastomer is one of the materials which is most widely used in the fields of automobiles, industrial machines, chemical plants, etc. A rubber elastomer is usually used as the material for members which, in use, are adapted to be brought into contact with various parts of machines and devices. These members include, for example, a control valve of an intake or an exhaust system or a fuel system of an automobile, a valve for use in other machines, O-ring, a gasket, a covering for a cable and a fixing roller of a copying machine. In connection with these uses, the rubber elastomers are required to have surface nonstickiness properties.
For rendering the surface of a rubber elastomer free of stickiness and low in friction, it has been proposed to modify the surface of the rubber elastomer. For example, there have been proposed a method in which the surface of a rubber elastomer is treated with fluorine gas (Japanese Patent Application Laid-open Specification No. 57-80039), and a method in which a rubber elastomer is treated with metallic sodium in liquid ammonia (Japanese Patent Application Laid-open Specification Nos. 57-56237 and 61-247744). However, the shaped article treated by such methods have an extremely hardened surface so that cracks are likely to occur when the shaped article undergoes deformation. In addition, the shaped article is still frequently unsatisfactory in its surface non-stickiness properties even after such treatment. Further, Japanese Patent Application Laid-open Specification No. 61-81437 discloses a method in which a shaped article comprising a vulcanized fluorine-containing rubber is treated with an amine to cause a predominant dehydrofluorination reaction to occur, so that the surface of the vulcanized shaped article is resinified, thereby obtaining a vulcanized shaped article having surface non-stickiness properties. However, due to the resinification, not only are cracks likely to occur in the shaped article, but also the article becomes poor in elongation. Therefore, such a shaped article is not suitable as a sealing material.
On the other hand, there have also been proposed physical surface treatment methods such as a fluororesin coating method and a high energy-etching method in which the surface of an elastomer is decomposed and volatilized by the action of plasma. However, the high energy-etching method cannot give a shaped article having a surface which is satisfactory in surface non-stickiness and in low friction properties. On the other hand, although a fluororesin coating method yields a shaped article having a surface somewhat improved in surface non-stickiness and in low friction properties, the treated surface is likely to be peeled off when the shaped article is used for a long period of time. Further, the method is also disadvantageous in that it is expensive. Beside the above-mentioned methods, there has also been proposed a method in which a low molecular weight component contained in a rubber elastomer is extracted with a solvent such as toluene. However, by such a method, a shaped article having a surface sufficiently improved in surface non-stickiness and in low friction properties cannot be obtained.
In these situations, it has been desired to develop a shaped elastomer article which is free from the above-mentioned drawbacks that inevitably accompany the shaped elastomer articles which have been treated by the above conventional methods.